Cellulosic fibrous structures, such as paper, are well known in the art. For example, cellulosic fibrous structures are a staple of every day life and are found in facial tissues, toilet tissue, and paper toweling.
One advancement in the art of cellulosic fibrous structures is cellulosic fibrous structures having multiple regions. A cellulosic fibrous structure is considered to have multiple regions when one region of the cellulosic fibrous structure differs in either basis weight, density, or both from another region of the cellulosic fibrous structure.
Multiple regions within a cellulosic fibrous structure can provide several advantages, such as economization of materials, increasing certain desirable properties and decreasing certain undesirable properties. However, the apparatus used to manufacture the multiple region cellulosic fibrous structure will greatly influence these properties.
Specifically a secondary belt, or comparable other apparatus, can affect the properties imparted to the cellulosic fibrous structure. As used herein, a "secondary apparatus" or a "secondary belt" refers to an apparatus or a belt, respectively, having an embryonic web contacting surface and which is used to carry or otherwise process an embryonic web of cellulosic fibers after initial formation in the wet end of the papermaking machinery. A secondary belt may include, without limitation, a belt used for molding an embryonic web of the cellulosic fibrous structure, a through-air drying belt, a belt used to transfer the embryonic web to another component in the papermaking machinery, or a backing wire used in the wet end of the papermaking machinery (such as a twin-wire former) for purposes other than initial formation. An apparatus or belt according to the present invention does not include embossing rolls, which deform dry fibers after fiber-to-fiber bonding has taken place. Of course, a cellulosic fibrous structure according to the present invention may be later embossed, or may remain unembossed.
As an example of how a secondary belt may input specific properties to a cellulosic fibrous structure, a wet molded and through-air dried cellulosic fibrous structure made on a secondary belt according to FIG. 4 of commonly assigned U.S. Pat. No. 4,514,345 issued Apr. 30, 1985 to Johnson, et al. may experience less curling at the edges than a cellulosic fibrous structure made on a secondary belt according to commonly assigned U.S. Pat. No. 4,528,239 issued Jul. 9, 1985 to Trokhan. Conversely, a cellulosic fibrous structure made on a secondary belt according to the aforementioned Trokhan patent may have a greater burst strength than a cellulosic fibrous structure made on a secondary belt according to FIG. 4 of the aforementioned Johnson, et al. patent.
This difference in performance relative to properties such as absorbency and burst strength may be attributed to the pattern of the drying belt used in wet molding and the through-air drying process to make the respective cellulosic fibrous structures. A cellulosic fibrous structure made on a secondary belt according to FIG. 4 of the aforementioned Johnson, et al. patent will have discrete high density regions and essentially continuous low density regions. Conversely, a cellulosic fibrous structure made on a secondary belt according to the aforementioned Trokhan patent will have continuous high density regions and discrete low density regions. This difference in the pattern of the regions influences other properties of the respective cellulosic fibrous structures as well.
For example, a cellulosic fibrous structure made on a belt according to the aforementioned Trokhan patent may have a lower cross machine direction modulus of elasticity and may have greater cross machine direction extensibility than a cellulosic fibrous structure made on a belt according to the aforementioned Johnson, et al. patent. However, these properties are typically offset by less sheet shrinkage and edge curling in a cellulosic fibrous structure made on a belt according to the aforementioned Johnson, et al. patent.
The caliper of certain cellulosic fibrous structures is closely related to the crepe pattern caused by the impact angle of the doctor blade. The doctor blade is used to remove the cellulosic fibrous structure from the surface of a heated Yankee drying drum and to crepe the cellulosic fibrous structure by foreshortening it in the machine direction. However, maintaining constant material properties (such as machine direction extensibility), which properties are influenced by the doctor blade is difficult. This difficulty is encountered because the doctor blade wears over time. Such wear is rarely constant over time, due to the taper of the blade and the stiffness of the blade changing as a third order power when wear occurs. Furthermore, the wear and changes which occur on one papermaking machine utilizing a particular doctor blade are often totally different than the wear and changes which occur on another papermaking machine using an identical doctor blade.
As the doctor blade wears, and the impact angle between the doctor blade and the Yankee drying drum becomes smaller, the cellulosic fibrous structure typically becomes softer, but loses tensile strength. Also, as the impact angle becomes smaller due to wear, the cellulosic fibrous structure may have greater caliper. Conversely, as the impact angle between the doctor blade and the surface of the Yankee drying drum becomes greater, such as occurs when the bevel angle of the doctor blade is increased, the doctor blade will typically wear at a faster rate.
But, the situation is even more complicated than described above. Not all secondary belts produce cellulosic fibrous structures which respond alike to changes in the impact angle of the doctor blade. For example, a cellulosic fibrous structure through air dried on a belt made generally in accordance with the teachings of commonly assigned U.S. Pat. No. 3,301,746 issued Jan. 31, 1967 to Sanford, et al. shows an increase in caliper as the doctor blade impact angle is decreased. However, the caliper generated on a cellulosic fibrous structure made on a secondary belt according to the aforementioned Sanford, et al. patent is not as great as the caliper of a like cellulosic fibrous structure made on a secondary belt according to the aforementioned Trokhan patent. But a disadvantage to the aforementioned Trokhan patent is that a cellulosic fibrous structure made thereon does not show a correlation to the doctor blade impact angle. Thus, one skilled in the art is forced to select between greater caliper generation and control of the caliper (and other properties) by adjusting the doctor blade.
Furthermore, wear of the doctor blade and the associated changes in impact angle cause different effects in cellulosic fibrous structures, which effects depend upon the pattern of the protuberances in the secondary belt. A cellulosic fibrous structure made on a belt having discrete protuberances will increase in caliper as the doctor blade wears, if the blade impact angle is not adjusted to compensate. Conversely, a cellulosic fibrous structure made on a secondary belt having a continuous pattern of protuberances is less sensitive to such wear.
It is not surprising that considerable effort has been expended in the prior art to achieve constant material properties by adjusting the impact angle of the doctor blades. In one example, illustrated by commonly assigned U.S. Pat. No. 4,919,756 issued Apr. 24, 1990 to Sawdai, the doctor blade is continually adjusted to minimize the effects of doctor blade wear on the material properties of the cellulosic fibrous structure.
However, adjusting the doctor blade requires more equipment, associated maintenance, and set-up time for the papermaking machinery than machinery which simply tolerates changes in the doctor blade impact angle. While, of course, it is desirable to produce paper having certain consumer desired properties, the art clearly shows a need for greater flexibility in the manufacturing process, and particularly a way to achieve greater flexibility by not having to adjust the doctor blade impact angle using complex machinery.
More importantly, the prior art shows a need for a secondary belt which generates relatively high caliper yet responds to changes in the impact angle of the doctor blade with like changes in the caliper of the cellulosic fibrous structures dried thereon.
As noted above, one way to achieve greater caliper is by adjusting the doctor blade. Another way to increase the caliper of a cellulosic fibrous structure having multiple regions is to increase its basis weight. However, this arrangement also increases the basis weight of other regions in which it may not be desirable to do so, requires greater utilization of fibers, and increases the cost to the consumer.
With the present invention, a way has been found to decouple the relationship between the Z-direction extent of the protuberances and the caliper of the cellulosic fibrous structure. Furthermore, other properties of the cellulosic fibrous structure may benefit from having been made on a secondary belt according to the present invention.
For example, another problem frequently encountered with cellulosic fibrous structures which try to minimize fiber utilization and present less expense to the consumer is pinholing. Pinholing occurs when regions of the cellulosic fibrous structure are deflected into the deflection conduits of the secondary belts and break through, so that an opening is present and light passes through the opening. Pinholing and transmission of light therethrough present a cellulosic fibrous structure having a less durable and lower quality appearance to the consumer, and is accordingly undesirable to the consumer.
One cause of pinholing in a cellulosic fibrous structure made on a belt according to the aforementioned Trokhan patent is caliper generation resulting from protuberances which are too great in the Z-direction. By generating caliper in this manner, Z-direction deflection of the cellulosic fibrous structure occurs to an extent that pinholing results. Thus, one using the aforementioned Trokhan belt is forced to select between caliper generation and reduced pinholing.
Other problems found in cellulosic fibrous structures made on a belt according to the aforementioned Trokhan belt of the prior art are cross machine direction shrinkage and curling of the edges of the cellulosic fibrous structure. Such shrinkage and curling are caused by structural movement during machine direction tensioning, such as inevitably occurs during winding and converting. Shrinkage requires a wider cellulosic fibrous structure for manufacture. Edge curling may cause fold over, leading to breakage of the web during manufacture. Both cause greater expense in the manufacturing process.
Unfortunately, the amount of shrinkage is also closely related to the amount of cross machine direction extensibility the cellulosic fibrous structure will undergo before rupture. While relatively greater cross machine direction extensibility is highly desired, due to allowing the cellulosic fibrous structure to elastically deform without tearing or shredding in use, the penalty for such desired cross machine direction extensibility is paid for at the time of manufacture by encountering greater cross machine direction shrinkage and curling.
Accordingly, it is an object of this invention to provide a secondary apparatus or belt which reduces occurrences of pinholing and shrinkage and curling of cellulosic fibrous structures during manufacture. It is an object of this invention to provide a secondary apparatus or belt which reduces occurrences of pinholing without requiring a corresponding reduction in the caliper of the cellulosic fibrous structure manufactured thereon. Furthermore, it is an object of the present invention to provide greater control over the caliper of the cellulosic fibrous structure with the impact angle of the doctor blade.